Lithium secondary batteries have been widely used as power sources of portable devices after they have emerged as small, lightweight, and high-capacity batteries since 1991. Recently, in line with the rapid development of electronics, communications, and computer industries, camcorders, mobile phones, and notebook PCs have appeared and undergone continuous and remarkable development. Accordingly, the demand for lithium secondary batteries as a power source for driving these portable electronic information and communication devices has increased day by day.
Lithium secondary batteries have limitations in that their lifetime rapidly decreases as charge and discharge are repeated. In particular, the above limitations are more severe at high temperature. The reason for this is due to a phenomenon that occurs when an electrolyte is decomposed or an active material is degraded due to moisture in the battery or other effects, and the internal resistance of the battery increases.
In order to address the above limitations, a technique of coating the surface of a cathode active material with an oxide of metal, such as magnesium (Mg), aluminum (Al), cobalt (Co), potassium (K), sodium (Na), and calcium (Ca), by a heat treatment has developed. Also, research to improve energy density and high-rate characteristics by adding TiO2 to a LiCoO2 active material has been conducted.
However, limitations, such as lifetime degradation or gas generation due to the decomposition of the electrolyte during charge and discharge, have not been fully resolved yet.
In the case that impurities are present in the surface of a cathode active material during a process of fabricating an electrode of a lithium secondary battery, the impurities may not only affect aging in a step of preparing an electrode slurry during the process of fabricating an electrode of a lithium secondary battery, but may also cause a swelling phenomenon in the lithium secondary battery by reacting with an electrolyte solution that is injected into the lithium secondary battery.
In order to address the above limitations, a method of coating the surface of a cathode active material with H3BO3 has been developed.
Examples of the above method may include a method of coating the surface of a cathode active material by mixing the cathode active material with H3BO3 by shaking several times using a shaker. However, in this case, H3BO3 particles may agglomerate on the surface of the cathode active material.
As another example, there is a method of coating a cathode active material by mixing the cathode active material and H3BO3 using mechanical compositing equipment, for example, a Nobilta™ device. In this case, since an amount of a coating layer included in the cathode active material is not increased when H3BO3 is added in a predetermined amount or more, there may be limitations in the reaction process.
Therefore, there is an urgent need to develop a method of preparing a cathode active material which may improve the performance of a lithium secondary battery while addressing the above limitations.